Verified answer

The dates already happened. They happen over a couple of evenings in the beginning of December. This year it was December 3rd and 4th. They close Disneyland early and then have the party after they clear out the regular day guests.

Sorry buddy but you missed it. It was on the 3rd & 4rd. Whenever the park closes at 6pm during December, you'll know that the cast members will be partying at 8pm. Luckily, DCA stays open a bit later that day for the park-hopping guest and regular guest. :)